Coal fly ash is produced as a by-product in the burning of pulverized coal. It is recovered from the flue gases. Fly ash is therefore a low cost material which is available in large quantity, but relatively few large scale commercial uses have been developed. With the increasing burning of coal to produce electrical energy, the amount of available fly ash can be expected to continually increase, and therefore processes using fly ash as a starting material will be increasingly important. The principal constituents of fly ash are the oxides of aluminum, iron, calcium, and silicon (Al.sub.2 O.sub.3, Fe.sub.2 O.sub.3, CaO, and SiO.sub.2).
The relative proportions of the metal oxides in fly ash vary with the type of coal being burned. In general, however, coal fly ash as produced will contain 15% or more by weight of aluminum oxide, and some fly ashes will contain as much as 28% Al.sub.2 O.sub.3. For example, in the United States, the burning of bituminous coal produces a fly ash having an average aluminum oxide content of about 22%, the range being from about 16 to 27% Al.sub.2 O.sub.3. The iron oxide content is usually much higher in bituminous coal fly ashes than with fly ashes produced from other types of coal, such as subbituminous lignite. Bituminous coal fly ashes may contain from 12 to 22% Fe.sub.2 O.sub.3. However, the iron rich particles of the fly ash can be separated to enrich the aluminum oxide content, such as by magnetic separation. Upgraded fly ash produced by initial removal of the iron oxide may contain as much as 18 to 21% Al.sub.2 O.sub.3.
It is known that fly ash can be leached with strong mineral acids to solubilize the aluminum, such as by leaching the fly ash with sulfuric acid. It is also known that fly ash can be reacted with phosphoric acid to convert at least part of the aluminum oxide to aluminum phosphate. A process has been proposed using this reaction as the first step in producing Al.sub.2 O.sub.3 or Al(OH).sub.3 and dibasic ammonium phosphate. Chem. Abst., 92, page 555: 179988c (1980).
Most of the recent research and development work on producing aluminum from fly ash has centered on the extraction of alumina by a lime-soda sinter process. After removal of the iron-rich magnetic fraction of the fly ash by magnetic separation, the upgraded fly ash is sintered with lime (or calcium carbonate), and soda ash. For efficient use on a production basis, this process may require the milling of the fly ash in admixture with the calcium carbonate and sodium carbonate, and then pelletizing or nodulizing of the ground mix prior to treatment in the sintering furnace at high temperatures. Following the sintering, cooling and further grinding are required prior to the leaching with a sodium carbonate solution, and additional filtration and precipitation steps are required to recover the solubilized alumina. Therefore, a need has been recognized for an alternative process for recovering aluminum from fly ash, but, as far as is known, it has not been suggested that the alumina might be recovered in the form of cryolite rather than as alumina (Al.sub.2 O.sub.3).